In reclaiming paper wastes, it is known art to employ various solvents for such purposes as ink removal, resins dispersion and removal of wax coatings. In some reclaiming operations, solvent residues are retained upon pulping of wastes which have been solvent treated. In other operations, the wastes are first pulped and the pulp is solvent treated. The bulk of the solvent is readily separated as a distinct phase but a small amount of it remains as a dissolved and/or dispersed residue in the treated pulp. Conventionally, these residues have been removed by methods such as distillation or by stripping with superheated steam, at a stage prior to dewatering of the pulp.
In the course of developing an improved method of employing solvents in waste paper reclamation, removal of solvent residues from dewatered pulps in conventional steam dryers employing rubber covered rolls was contemplated. The solvent residues constituted such a small proportion (about 170 ppm) of the dewatered pulps that no substantial solvent swelling (and softening) of the rubber was anticipated. However, laboratory tests showed that the roll covers would be damaged to an intolerable extent if the solvent contents of the pulps were not reduced prior to drying. That is, the useful life of the roll covers would be reduced so much that the resulting technical and/or economic disadvantages would outweight whatever advantages might be realized from the reclamation operation contributing the solvent residue to the pulp.
Gas stripping is a well-known method of removing volatiles from water and aqueous solutions but no suggestion of using this technique with slurries was found in the literature. Paper pulps, i.e., dispersions of cellulosic fibers, constitute a rather unique type of slurry, in that strong secondary bonding forces come into play when the concentration of fibers in a given volume of pulp exceeds that corresponding to a consistency of about 0.4 weight percent solids. At higher consistencies, the tendency towards inhomogeneity, i.e., clumping and settling, increases notably and can be overcome only by agitation intense enough to provide high shear rates. (It is known that pulp homogeneity can be maintained at consistencies as high as about 3 percent by devices such as "hydropulpers," but only at the expense of very considerable power inputs.) Thus, it could be expected that adequate dispersion of a gas in a commercial paper pulp might not be achievable without uneconomic power expenditures.
Attempts by the present applicant to establish intimate contact between air streams and intensely stirred pulps having consistencies in excess of about 1.5 percent were futile. Dispersion of air in the pulps could not be achieved and cavitation and channeling resulted.
There is also the consideration that even in pulps having consistencies as low as about 0.40-0.50%, a substantial degree of interaction between cellulosic fibers is still apparent. In fact, paper forming on a Fourdrinier screen requires a consistency (0.4-0.5%) such that this interaction will result in uniform matting as drainage proceeds (but without the occurrence of clumping prior to deposition on the screen). Thus, even at a pulp consistency of 0.4%, the existence of a uniform network of fibers within which relative motion between fibers is hampered by fiber-fiber and fiber-water attractive forces, is indicated. For this reason, it was considered possible that solvent droplets would be effectively "trapped" between closely spaced or touching portions of fibers. Additionally, interference with propagation in the liquid phase of the turbulent eddies believed essential to efficient mass transfer by diffusion could be anticipated. Therefore, even though relatively good dispersion of the stripping gas in the pulp were achieved, further reduction of solvent contents below already low (residual) levels still might require the use of excessively large volumes of stripping gas.
Preliminary dilution below the 0.4% level is of course contraindicated in view of the cost of reconcentration (after stripping).